Jet engines used in aerospace applications require periodic maintenance and repair. Typically, such jet engines are gas turbine engines surrounded by a nacelle. Part of the gas turbine engine surrounded by the nacelle is a core that includes fan, compressor, combustor, and turbine sections. A bypass duct passes through the gas turbine engine, and fan blades pass through the bypass duct. The core generates power that is used to propel an attached aircraft. The core is used to drive fan blades in the bypass duct to generate thrust, and core exhaust also creates thrust to propel the aircraft.
In order to facilitate maintenance and repair of the engine, known nacelles include doors that open outwards from the side of the nacelle, called “D-doors”. When the engine needs repair or maintenance, the D-door is opened to provide access to engine parts. Some of the engine components that need regular maintenance or repair include the core and core externals. D-doors typically provide access to components of the core such as the combustor and turbine exhaust case that are not accessible from either the upstream or downstream ends of the gas turbine engine. Core externals include those devices that support the functions of the core, such as oil supply and drain, fuel supply, sensors, and wiring and connections to the sensors.
Externals pass through the bypass duct of the gas turbine engine. For example, fuel lines, oil supply and drain lines, and sensor leads must be connected to fuel tanks, oil supply systems, and controllers that are outside of the nacelle, respectively. Often, these externals are not suitable for routing through the bypass duct unprotected. Externals are often not structurally capable of supporting the loads that would be applied on them in the bypass duct. Furthermore, externals are often not aerodynamic, and routing through the bypass duct would result in undesirable drag on the bypass airstream. For this reason, externals are typically routed through a bifurcation, commonly referred to as a “bi-fi.” A bi-fi is typically shaped as an airfoil having low to zero camber, and a chord direction parallel to the direction of the bypass airstream. The airfoil that makes up the bi-fi is hollowed out such that externals may be routed to the pylon or other sections of the aircraft without passing through the bypass airstream unprotected.
A common design of gas turbine engine has both an upper bi-fi and a lower bi-fi. The upper bi-fi shelters externals passing between the core and the pylon on which the engine is mounted. The lower bi-fi may be used for additional externals, or may be present to provide aerodynamic symmetry to the bypass duct.
D-doors are often arranged at or near the mid-point, axially, of the nacelle in which they are housed. D-doors often open upwards in the manner typically described as a “butterfly door.” By opening the D-doors, a mechanic can gain access to the externals and/or core of the engine housed in the nacelle behind the D-door. Because the externals are housed not only within the nacelle (i.e., behind the D-door) but also within the bi-fi, known bi-fi designs are split such that they can also open in the “butterfly door” manner, or removed entirely. In other words, known bi-fi constructions include two identical halves, each half a mirror of the other side, which may be attached to one another to form a single airfoil surrounding the core externals of the gas turbine engine.